It is generally known that low temperature polymers can be made more heat stable by blending such polymers with higher temperature polymers to form a polymer composite or alloy. Polymer composites are typically prepared from materials of two separate origins by dispersing one solid phase in a continuous matrix of another phase. Mostly, polymer composites consist of a base polymer reinforcing fibers, fillers, and/or whiskers. In addition, polymer composites may contain additives such as plasticizers, colorants, flame retardants, as well as stabilizers against heat and/or sunlight.
In the preparation of polymer blends, in addition to one phase being fluid as with conventional composites, the second phase can also be fluid, either as a melt or as a polymerizing monomer. Also, unlike conventional polymer composites, in blends, phase reversal or inversion may be achieved, depending on the relative concentrations and viscosities of the two polymers. Thus, from a state wherein one component is continuous in phase, a polymer blend can comprise a system which is continuous with respect to the second phase or one in which both phases are continuous such that one component can become enclosed in the second component and vice versa. Accordingly, where the properties of the two polymers are widely different, extreme changes in mechanical behavior can be experienced in the resulting blend.
Blends and alloys consisting of a combination of two or more polymeric resin systems where at least one of the polymers is present in a concentration greater than 5% by volume are well known to the art. As stated above, blends are mixtures of two or more resins which are blended, customarily in the molten state, to form new materials. Unlike copolymers, grafts, or interpenetrating polymer networks, no chemical synthesis or formation of new covalent bonds need occur. Blends have been designated as either miscible or immiscible depending upon the number of phases present.
Miscible or soluble blends comprise one phase with one glass transition temperature (Tg), wherein individual polymer segments are intimately blended with some specific chemical or physical attraction taking place between dissimilar polymer chains, e.g., hydrogen bonding or donor-acceptor. In contrast, immiscible blends consist of two or more discrete phases (continuous and disperse) of two or more Tgs. Completely immiscible blends have limited product potential because of lack of adhesion at the polymer interface. Compatibilizers may be added to such blends to make useful alloys.
Most commercially-marketed resin alloys are formed via some type of melt mixing utilizing a continuous-type intensive mixer or an extruder. In this process, two or more polymers in pellet or powder form are generally premixed or metered into an extruder (a single screw or a multiscrew), or into a continuous-type intensive mixer, fluxed for a brief period, and then shaped into pellets from strands or diced from sheets.
Inorganic glasses can exhibit many desirable properties; for example, high elastic modulus, abrasion resistance, stain resistance, thermal stability, inertness to solvents, low coefficient of thermal expansion, and low permeability to moisture and gases. On the other hand, organic polymers which are generally known to be poor in the above properties, can demonstrate such advantageous characteristics as high elasticity, flexibility, toughness, light weight, and ease in shaping, which properties are generally lacking in inorganic glasses.
Glass/polymer blends attempt to combine the properties of inorganic glasses and those of organic polymers. Glass polymer composites, which may be regarded as multi-phase materials of two or more components in which the polymer comprises the continuous phase, can be considered as containing glass fillers or reinforcing agents. Filled plastic products customarily consist of organic polymers enveloping discrete organic or inorganic particles, flakes, fibers, whiskers, or other configurations of materials. These filler materials may be incorporated principally for the purpose of reducing the overall cost of the product without seriously undermining the properties of the polymer. Similarly, the filler materials may be included to impart some improvement to a particular physical property exhibited by the polymer. For example, ceramic and glass fibers have been entrained in polymer bodies to provide reinforcement to the composites. The strength demonstrated by those products is primarily dependent upon mechanical bonding between the inorganic fibers and the organic polymers as well as alignment of the reinforcement in the test direction.
In recent years, composite bodies consisting of inorganic glasses exhibiting low transition temperatures and organic polymers have been disclosed which exhibit the combined properties of glasses and polymers. For example, U.S. Pat. No. 3,732,181 describes seven general methods by which glass in the form of fibers, films, flakes, powders, or sheets is combined with a polymer to form a composite mixture which can be formed into a desired configuration through a variety of shaping means. As disclosed therein, the ratio of polymer-to-glass may range from 0.1:99.9 to 99.9:0.1 on a volume basis, but more typically, about 5-66% by volume. The reference also discloses three broad glass compositions exhibiting properties which render the glasses suitable for use in glass-plastic composite articles, namely:
(a) PbO+P.sub.2 O.sub.5.gtoreq.95 mole %, wherein PbO constitutes 20-80 mole %; PA1 (b) PbO+R.sub.2 O (alkali metal oxides)+.gtoreq.95 mole %, wherein PbO comprises 5-60 mole %, R.sub.2 O constitutes 5-35 mole %, and P.sub.2 O.sub.5 is present up to 85 mole %; and PA1 (c) PbO+R.sub.2 O+B.sub.2 O.sub.3 +P.sub.2 O.sub.5.gtoreq.95 mole %, wherein PbO comprises 5-30 mole %, R.sub.2 O constitutes 5-30 mole %, B.sub.2 O.sub.3 comprises 5-20 mole %, and P.sub.2 O.sub.5 makes up 15-85 mole %. PA1 (a) high shear dispersive mixing of finely-divided bodies of an inorganic glass and at least two organic thermoplastic or thermosetting polymers at a temperature and viscosity represented by the working temperature of the glass and polymers to form a glass polymer mixture; and PA1 (b) shaping the mixture into an article of a desired configuration. PA1 "high glass loading" means that the blend contains at least 65 wt. % glass; PA1 "good dimensional stability" means that the blend exhibits thermal expansion in three directions (x,y,z) no greater than 95 .mu.m/m .degree. C., preferably less than 90 .mu.m/m.degree. C.; also, the ratio of the thermal expansion in the x,y directions is less than 2.0, and more preferably in the range of 1 to 2.0; PA1 with respect to glass, "essentially non-hygroscopic" indicates that the glass component will demonstrate a weight gain of less than 1.times.10.sup.-6 grams/cm.sup.2 /minute when exposed at 40.degree. C. to a relative humidity of 80%; PA1 "blend" and "alloy" are used as such terms are defined by Leszek A. Utracki, in Polymer Alloys and Blends, (1990), Part 1, pgs. 1-3; thus, a "polymer blend" is a mixture of at least two polymers or copolymers, and an "alloy" is an immiscible polymer blend having a modified interface and/or morphology; PA1 "excellent mechanical properties" means that the blend, among other things, exhibits thermal expansion in three directions (x,y,z) no greater than 95 .mu.m/m .degree. C., preferably less than 90 .mu.m/m.degree. C.; also, the ratio of the thermal expansion in the x,y directions is less than 2.0, and more preferably in the range of 1 to 2.0; and PA1 sometimes the term "tri-blend" is used broadly for the sake of brevity to describe the inventive blend and is used to indicate that the blend contains at least three components--glass and at least two polymers; this use is intended to distinguish the present blend from known single polymer glass blends. In reality, the inventive blend can contain more than three components as in blends containing glass and three polymers.
U.S. Pat. Nos. 3,885,973; 3,935,018; 3,964,919 and 3,926,649 disclose glasses which may be suitable for co-processing with organic polymers to form composite articles of the type discussed in detail in U.S. Pat. No. 3,732,181 supra.
Recently, U.S. Pat. No. 5,043,369, herein incorporated by reference, has disclosed a glass/polymer blend wherein the glass phase and the polymer phase are co-continuous, with the particles of each phase being simultaneously enclosed within larger regions of another phase (i.e., localized phase inversion/reversal.) In this patent, the glass and polymer demonstrate at least partial miscibility and/or a reaction there-between such that the two components are intimately blended together. It is believed that the blend results in the formation of a compatibilizing component in-situ to yield an alloy. Also, the glass/polymer blend of this reference exhibits an essentially uniform, fine-grained microstructure wherein the glass and polymer elements comprising the microstructure are of relatively uniform dimensions. The reference patent discloses a phosphate-based glass within two general narrow composition regions which are essentially non-hygroscopic and exhibit good resistance to chemical and moisture attack, and which can interact with a variety of polymers to produce alloy articles.
More recently, U.S. Pat. No. 5,328,613 (Beall et al.) disclosed semi-permeable microporous polymer bodies made from durable high-temperature thermoplastics and subsequently leaching the glass out of the polymer to create a continuous polymer network. Also, U.S. Pat. No. 5,367,012 (Aitken et al.) has recently disclosed an alloy comprising at least one organic thermoplastic or thermosetting polymer, at least one phosphate glass, and a water soluble stabilizer to provide a source of metal cations.
There continues to be a need for cost-effective, durable, flame-retardant glass polymer compositions having essentially uniform, fine-grained microstructure wherein the glass and polymer elements comprising the microstructure are of relatively uniform dimensions. There also continues to be a need for composite articles having a high surface smoothness. For such articles, the particle size must be as small as possible, preferably on the order of micron scale or less.